Processes for carbon-impregnating glasses in order to impart electrical conductivity thereto are known. U.S. Pat. No. 2,556,616, for example, describes the impregnation of porous glasses with soluble carbohydrates, particularly sugars, followed by drying and firing to convert the sugars to a continuous conductive carbon phase. Problems attending the use of sugar solutions for this purpose include those relating to the high viscosity and low stability of the solutions, and to the difficulty of obtaining uniform and reproducible carbonization of the sugar impregnant on firing the impregnated glass.
U.S. Pat. No. 3,813,232 describes a method of providing conductive porous glasses which comprises impregnating the porous glass with acetophenone and sulfuric acid, and thereafter heating the impregnated glass to decompose the impregnants and provide a conductive carbon phase in the glass. Disadvantages of this process include the hazards associated with the handling of hot (100.degree.C.) organic and sulfuric acid solutions, and the evolution of copious amounts of noxious sulfur-containing compounds from the glass on heating.
A more convenient method of providing a continuous carbon phase in a porous glass comprises the use of polymerizable furan derivatives as impregnants for this purpose. U.S. Pat. NO. 3,775,078 describes the production of carbon-containing porous glasses exhibiting increased refractoriness and also electrical conductivity, by impregnating a porous 96% silica glass with a solution of furfuryl alcohol in a suitable solvent, polymerizing the furfuryl alcohol in situ in the pores of the glass to provide a resin, and firing the glass in a non-oxidizing atmosphere to convert the resin to carbon.
In attempting to adapt the furfuryl alcohol method to the production of electrical resistance elements, serious difficulties were encountered in obtaining reproducible resistance values, even among resistors processed in the same lot or taken from the same section of resistor cane stock. It was recognized that a broad range of resistance characteristics, including room temperature DC resistivities ranging from about 10.sup.0.01 -10.sup.10 ohm centimeters, could be obtained in the product by controlling the amount of furfuryl alcohol and thus the amount of carbon introduced into the pore structure of the glass. However, considerable variations in resistivity were observed, particularly in the higher resistivity range of about 10.sup.1.5 - 10.sup.10 ohm centimeters, despite attempts to reduce these variations through careful control of glass pore characteristics, temperature, impregnating solution composition, impregnation time, drying schedules, and firing treatments.
It is a principle object of the present invention to provide an improved process for the production of electrical resistive elements using furfuryl alcohol to provide a continuous carbon phase in a porous glass, which permits significantly better control over the resistivity of the product.
It is a further object to provide high resistivity electrical resistance elements produced in accordance with this improved process.
Other objects and advantages of the invention will become apparent from the following description thereof, and from the appended DRAWING which comprises a graph showing the relationship between the electrical resistivity of a carbon-impregnated porous glass produced by impregnation, polymerization, and carbonization of a furfuryl alcohol impregnant and the concentration of furfuryl alcohol present in the solution utilized to impregnate the porous glass.